Conventional rotary slide valves for use in a power steering system have a housing, an input shaft and an output shaft which can be rotated relative to each other to a limited extent, a centering piston which is non-rotatably and axially movably connected with the input shaft and delimits a pressure chamber in the housing, a centering ring which is non-rotatably connected with the output shaft, and an inlet to the pressure chamber.
Such an arrangement is known from European patent document 0 551 619 B 1, and U.S. Pat. No. 5,339,917 which is hereby incorporated by reference. The centering piston serves to provide a centering or restoring force, i.e. to urge the input shaft and the output shaft relative to each other into a neutral position in which no power assistance is provided. In the known construction, the centering piston is urged against the centering ring by a pressure spring. Unless any further pressure forces act on the centering piston, the same thus generates a firm centering dependent on the spring action. If a stronger centering is desired, for instance at high driving speeds, pressure is applied on the pressure space, so that a stronger centering effect is achieved. As a result of this stronger centering effect, a higher torque must be applied on the steering wheel connected with the input shaft, in order to obtain a defined rotation between input shaft and output shaft and thus of the rotary slide valve. If, on the other hand, a particularly small steering moment is desired, in order to generate a predetermined power assistance, for instance when parking the vehicle during standstill, it is provided according to the known construction to apply pressure on the side of the centering piston opposite the pressure space, in order to generate a force acting against the spring force.
It is the object underlying the invention to develop a power steering system as mentioned above such that with a simpler mechanical structure largely any desired centering effect can be caused.